The kinetics of oxidation of human oxyhemoglobin by ferricyanide were studied. With the aid of the conclusion of Antonini et al. (Biochemistry 4: 545, 1963) that ferricyanide oxidizes only deoxyHb, the effect of pH and ionic strength on the fraction of deoxyHb and on the electrostatic attraction or repulsion of the ferricyanide ion was used to calculate variations in the rate of oxidation. At low pH the agreement with experiment was fairly good even though factors difficult to evaluate, such as protein unfolding and ion binding, were ignored. At high pH the measured rate is much faster than the calculated rate, suggesting that at high pH other mechanisms involving less electrostatic repulsion become more important. The reactions of formaldehyde with amino acids, nucleosides, and similar compounds were studied. The primary reaction, replacement of an active hydrogen by an hydroxymethyl adduct, may be followed by intermolecular methylene bridge formation if the concentration of reactants is high, or by intramolecular methylene bridge formation if steric factors are favorable.